As performance requirements for pneumatic tires are geared toward high-performance vehicles, a tire is required to have excellent grip performance when traveling on dry road surfaces and wet road surfaces as well as excellent fuel efficiency performance so as to accommodate increasing concerns regarding global environmental problems. As a result, by compounding silica in rubber compositions that form tread portions, dynamic viscoelastic characteristics of the tread rubber such as loss tangent (tan δ) and the like have been improved, heat build-up has been suppressed, rolling resistance reduced, and fuel consumption performance improved, while wet performance has been improved. However, silica tends to cause poor dispersion due to a lack of affinity with diene rubbers, and the expected effect of improving low rolling resistance and wet performance has not necessarily been sufficiently achieved. When the compounded amounts of silica and reinforcing fillers are reduced in order to improve the poor dispersion of silica and to ensure low heat generation, this induces a reduction in not only tire rigidity, but also dry grip performance and wet grip performance, so it has been difficult to achieve all of these performance requirements.
To resolve this problem, Japanese Unexamined Patent Application Publication No. 2009-091498A proposes improving the dispersibility of silica by compounding silica in a rubber composition with a terminal-modified solution polymerization styrene butadiene rubber where the terminals are modified by a polyorganosiloxane or the like, thereby reducing heat build-up (tan δ at 60° C.) and enhancing wet grip performance (tan δ at 0° C.). On the other hand, there have also been attempts to increase the silica compounded amount so as to achieve both low rolling resistance and wet grip performance by using various silane coupling agents in combination. However, when the compounded amount of a terminal-modified solution polymerization styrene-butadiene rubber or silica is increased, the viscosity of the rubber composition increases, which leads to the problem of deteriorated workability.
Accordingly, it has been difficult to dramatically improve the low rolling resistance, dry performance, and wet performance while maintaining excellent workability.